Chris Impey is a distinguished astrophysicist who observes quasars and distant galaxies, and is a former vice-president of the American Astronomical Society. Recently he added popular science writing to his interests. His first book, The Living Cosmos (2007), was an astronomer's look at life on Earth, and how what we have learned can be applied to the Universe as a whole. How it Ends takes us to the opposite extreme of the time axis: instead of considering origins, Impey invites us to contemplate death in a cosmic landscape. This secular eschatology is telescopic in its ambition, focusing by turns on the ultimate fate of the individual, of species, the solar system, the Milky Way and the entire Universe.
We already know remarkable facts about life, the Universe and everything. We know that the Universe exploded from a quantum quirk to a cosmic zoo of 50 billion galaxies. We understand how species have evolved and developed from molecular goo to millions of species, only one of which is capable of turning the Universe into an intellectual puzzle. Impey's elegant enquiry takes us from the past to the future: what happens next?
The project starts at home, by reviewing our own mortality. There is not much scope for storytelling here. But my interest picked up in chapters on the future of humanity and threats to us as a species. Here, Impey offers some effective messages by looking at large numbers. Are humans a successful species? The answer depends on the metric you choose. There are 6.8 billion of us (2 million of whom are at any one time 10km above the Earth in aircraft). That is too many, according to the futurologists who fear a Malthusian disaster. But consider Antarctic krill, the tiny crustaceans that are an important component of the ecosystem. They each weigh about a gram, but if you scooped them all up, the combined mass would exceed that of all humans. Successful species on land include ants, which have been around for more than 100 million years and now boast 22,000 species. In keeping with the book's title, the author sounds a familiar note of caution: uniquely among life, only humans have the capacity to destroy their species by triggering a global environmental disaster or a nuclear holocaust or by launching biological terror.
The future of humanity is uncertain on long-term horizons, so it is difficult for any writer or visionary to offer insights into how humanity may cease to exist. In her autobiography, My Animal Life (2010), the novelist Maggie Gee reflects that our home planet existed for 4 billion years before our birth and it will survive as a planet for 4 billion years after our death. Although the Earth has all the ingredients of life in abundance, our own existence is a puzzle in the sense that we haven't been here very long.
So let's bring on the astrobiologists and set our existence in a universal setting. Our environment has the potential to end our tenancy. Impey takes a thorough look at the risk from asteroids: the probability of a killer rock is low, but a small probability multiplied by millions of years eventually stacks up. He dips into natural biological hazards, too: mass extinctions, bacterial poisoning, green goo and so on.
If the Earth were to become unavailable, for whatever reason, where can humans go? Here, Impey serves up fare that will be familiar to astrobiologists, but less well known outside that discipline. The discussion considers a pre-Copernican stance, namely that we are special and planets like Earth are rare. Nevertheless, we should look elsewhere. But just suppose we were to find evidence of past life on Venus, Mars or Titan: would that help us to figure out the future of our own species?
The second part of the book places everything in a cosmological setting. The Sun shines for another 5 billion years. Then what? From here, the narrative strays into science fiction and transhumanism. The story borrows - and why not? - from science fiction author Arthur C. Clarke and others, using the tricks of continuous compound growth to imagine super-intelligence and magical technologies. Impey takes the responsible line by pointing out that speculation on future technology and the dream of interstellar colonisation is pure fantasy.
Astrobiologists who are interested in exobiology and the search for extraterrestrial intelligence should read this wonderful book carefully. If humanity has only a short lease of life, then what can we learn from that about life elsewhere? I get the impression that a case is slowly building that simple life forms may be common but that we are unique. We have no companions with whom to communicate.
How it Ends: From You to the Universe
By Chris Impey
W.W. Norton, 352pp, £18.99
Published 4 July 2010